Our mechanical engineer explains how said fuelsaving measure works.
THERE is an incredible amount of engineering that goes into the development of fuel-saving measures in automobiles, such as the coast mode for certain automatic transmissions. The ZF 8-speed automatic, for example, has a special hydraulic pressure accumulator that makes it possible to disengage (and later re-engage) Drive when the driver lifts his foot off the accelerator pedal and the car is coasting on a descent, or coming to an eventual stop on level ground. Drive can be re-engaged immediately when the driver’s right-foot demand for acceleration is detected. Any automatic transmission without this feature can be manually shifted to N (Neutral) to simulate coast mode. But without the benefit of backup oil pressure, the result is premature wear on the gear-ratio selector clutches. Hence, the practice should be avoided, even if it saves petrol.
But with automated dual-clutch gearboxes, such as Volkswagen’s DSG, Porsche’s PDK, Audi’s S tronic and Mercedes’ 7G-DCT, an electric oil pump provides the hydraulic pressure to engage Drive on engine restart at a traffi c light, or to switch between Drive and Neutral for coast mode. With any dual-clutch gearbox, whether or not the car comes with auto stopstart and/or coast mode, there is no adverse eff ect in shifting manually to Neutral while coasting.
This is because for such gearboxes, power transmission does not rely on positive hydraulic pressure, as is the case with the automatic transmission. Instead, the hydraulics are for the purpose of mechanically releasing the clutches or shifting gears. There is no risk of any damage if you choose to select N when “freewheeling” downhill. You can do the same with any manual gearbox. Coast mode can reduce the car’s fuel consumption by a few percentage points, depending on the number of coasting opportunities and distances “coasted”.
